A508-Alloy 52與316L-Alloy 52異質金屬銲件之高溫水環境應力腐蝕特性研究

Abstract

The welding of dissimilar metals is widely used for joining low alloy to stainless steels at several locations such as pipe and nozzle joints in nuclear reactor pressure vessels. Nickel- based alloys, such as Alloy 52, are often applied in dissimilar metal welds (DMWs) as filler metals to reduce differences in physical, metallurgical and mechanical properties between the involved materials. In this study, the susceptibility to the stress corrosion cracking (SCC) of the A508-Alloy 52 and 316L-Alloy 52 welds in high-temperature water were evaluated by using constant extension rate tensile (CERT) test with notched specimens. Furthermore, the TIG welding process, filler metal selection (Alloys 52 and 52M) and microstructure of the DMWs were also investigated. Experimental results indicated that the heat-affected zone (HAZ) of the A508 side of DMWs consisted of a mixture of bainite and ferrite. For single-pass welds, the grain size in the HAZ of A508 side was considerably large and the structure was mainly bainite. However, the HAZ comprised mostly ferrite and fine grains in the multi-pass welds as a result of multiple weld thermal cycles during the process. The use of temper bead technique eliminated the need of post-weld heat treatment (PWHT) and lowered the HAZ hardness of the A508 side. Such a technique is particularly suitable for field repair and renovation. In the weld metal, martensite and Type II boundaries were observed in the transition zone adjacent to the weld interface of Alloy 52/A508, which may cause corrosion related failures in service. On the other hand, the HAZ of the 316L side showed neither microstructural change nor sensitization after a PWHT at 621°C/24 h. The transition region of the Alloy 52/316L weld metal was susceptible to hot cracking which could be reduced by lowering heat input in welding. Moreover, the weld overlay of a 309L buffer layer prior to deposit Alloy 52 on the 316L substrate significantly reduced the hot cracking susceptibility and could tolerate the substrate with higher contents of S and P. The results of varestraint tests demonstrated that Alloy 52M had better hot and ductility-dip cracking resistances than Alloy 52. As a result, it is recommended that Alloy 52 should be replaced by Alloy 52M for welding A508-316L and that kind of components in the nuclear power plant system. The results of the constant extension rate tensile (CERT) tests in 300°C water revealed that the notched round-bar specimen with a circumferential notch at various locations of the DMWs was useful in evaluating the SCC behavior of a narrow region in the welds. In the weld metal of A508-Alloy 52 welds, the relative susceptibility to SCC in terms of the ductility loss in increasing order of severity was as follows: the undiluted weld metal, the transition zone and the weld interface. SEM fractographic observations were consistent with the SCC results, i.e., an increased ductility loss or SCC susceptibility was associated with more brittle fractures. Apparently, the presence of Type II boundaries caused intergranular cracking and significantly reduced the SCC resistance of the weld in 300°C water. The structural discontinuity at the interface also increased the SCC susceptibility of the weld interface specimen.The test results of the A508 HAZ specimens clearly indicated that the lower welding current was beneficial to the SCC resistance,which was in terms of the loss of notched tensile strength, in the HAZ of A508 steel. In multi-pass welds, the use of a low heat input resulted in a better SCC resistance than that of a high heat input due to the existence of a more refined microstructure in the HAZ. Additionally, Alloy 52 weld metal also revealed better SCC resistance than either the 316L base metal or the weld interface of Alloy 52/316L.